1. Field of the Invention
The present invention relates to a liquid crystal display for reflection operation with a guest-host liquid crystal layer disposed between two cell plates and respective front and rear electrodes, and with a diffusely divergent reflector integrated in the interior of the cell, at least the front side of the cell plate and the front electrode being transparent. The invention also relates to a method of producing this liquid crystal display.
2. Description of the Prior Art:
The principle of cholesterinic guest-host liquid crystal displays is known (Appl. Phys. Lett. 13, 91, 1968) and has already been used for constructing electro-optic devices (see DT-OS 2,410,557). From Journal of Applied Physics, Vol. 45, 1974, p. 4718-4723 a liquid crystal display device is known which utilizes pleochroic dyes (guest) and cholestorinic liquid crystal mixtures (host). This liquid crystal display is operated by reflection and has excellent brightness since external polarizers are not used. This publication explains the basic principle of the guest-host effect (for example, FIG. 2) and gives information regarding suitable liquid crystal mixtures and dyes and measurement results obtained by experiment.
Although it has been shown that in liquid crystal displays having a guest-host liquid crystal layer it is possible to dispense with expensive polarizers, it has not hitherto been possible for this type of display to compete with the successful nematic rotary cells. It has been found that in guest-host displays operated by reflection, reading parallax occurs through a reflector disposed behind the liquid crystal cell. This has a particularly disturbing effect in relatively small displays in which the width of the electrode elements is smaller than or comparable to the thickness of the cell plates used. Dimension ratios of this kind often occur in displays which are intended for use in wristwatches or digital voltmeters.
It therefore appears obvious to integrate the reflector in the interior of the cell. Unfortunately, however, new disturbing effects then occur, particularly in alphanumeric displays. In this case the integrated electrically conductive reflector performs the function of a rear electrode and results in not only the activated electrode elements themselves but also the connecting lines leading to them becoming visible. Since for technological and electrical reasons there is a limit to the narrowness of these connecting lines, this disturbing effect prevents the appropriate use of displays of this kind.
As had already been done in the production of nematic rotary cells, both the front and the rear electrode have hitherto also been made by a thin film technique for guest-host liquid crystal displays. Thus, for example, cell plates made of glass were vapor-coated on one side in a high vacuum with a conductive transparent layer of SnO.sub.2 or In.sub.2 O.sub.3 of a thickness of 2000-5000 A. On these cell plates, referred to as conductor glasses, the electrode pattern corresponding to the nature of the characters to be displayed and to the activation system of the display was then printed on the coating side with acid-resistant protective lacquer by a screen printing process. This was followed by an etching process, for example etching in a solution of HCl or H.sub.3 PO.sub.4, in which the excess parts of the coating were etched away and the coating cleaned by rinsing a number of times in water.